Extrusion die apparatus employed for the extrusion of thermoplastic material, more particularly foamable thermoplastic material, such as, for example, but not limited to, olefinic materials like polyethylene or polystyrene, employ inner and outer die lips. The inner and outer die lips have a one and the other end, and at the one end form a concentric die gap for the extrusion of a concentric thermoplastic material, and which inner and outer die lips form therebetween a thermoplastic flow passageway, generally extending axially through the inner and outer die lips. The flow passageway at the one end is usually angularly disposed upwardly, and which angular upward flow area includes a choke area, which comprises a cross-sectional flow restriction area for the extruded thermoplastic material. Choke areas are particularly employed in connection with extrusion die apparatus to provide for the mixing and distribution of the hot, melted, extruded plastic as it passes from the inlet of the extruded die apparatus toward the die gap. Some die apparatus employ extrusion passageway sections which have designated spider legs and which tend to produce spider marks in the extruded thermoplastic material if the choke area is not employed to compress the extruded thermoplastic material for better distribution.
Generally, the choke area is tapered slightly to restrict the extruded thermoplastic material, such as the foamable material, with minimum increase of pressure drop, since a large increase in pressure drop might cause prefoaming within the die passageway, where foamable thermoplastic material is being extruded. The choke area is concentric where the extruded thermoplastic material is to be concentrically extruded through a concentric die gap, so that the choke passageway, cross-sectionalized, is generally uniform, otherwise there is unequal extruded thermoplastic flow material, so that the flow is not concentric from the die gap. Generally, in order to provide for a concentric flow, a threaded bolt or screw adjusting means is employed adjacent the outer die lip to permit peripheral adjustment of the position of the outer die lip about the inner die lip through a plurality of bolts on the outer die lips. In the past, certain choke rings have, in fact, been used to adjust the choke area or gaps. However, such choke rings have not proven satisfactory, and generally have failed to provide for the accurate control of concentricity of the flow through the choke area.
It is therefore desirable to provide for a new and improved extrusion die apparatus particularly for use with foam sheet extrusion, and an extrusion die system and method containing an easily adjustable choke gap and which permits adjustment also of concentricity of the choke gap.